Commercially available pipe typically is manufactured to a nominal outer diameter which varies plus and minus depending on the tolerance range. Collets are used to hold or clamp pipes during, for example, welding of the pipes. Welding can be accomplished with, for example, an orbital weld head of the type shown generally in U.S. Pat. No. 4,379,215, the entire disclosure of which is hereby incorporated by reference. A collet for a given pipe size needs to provide enough compliance to securely hold pipes that have a size anywhere within the given tolerance range for that pipe size. The collet typically must fit certain space constraints. Some prior art pipe collet designs use integrally machined components to provide spring-like compliance, but these designs can be inconsistent and relatively expensive, and can require mating collet components that also are relatively expensive.
The use of rigid collets for holding the pipe during the welding operation is much preferred as compared to split and/or adjustable collets or similar holding devices. The reason for this is that during the welding operation, thermal stresses tend to cause the pipe to move creating misalignment between the two sections. The movement is greater and/or more likely to happen with the split collets and the adjustable collets or holders. With respect to the solid or rigid collets, however, there are problems in assuring that pipe throughout the range of standard commercial tolerances can be held properly. That is, a typical commercially available pipe used for fluid systems and the like, has, for example, a nominal outside diameter of 3.000 inches which varies ±0.030 inches. It has been difficult to compensate for the diameter variations which result from the tolerance variations and, at times, it has been difficult to properly hold the pipe during the welding operation. Also it has at times been difficult to hold out of round pipes or tubes in the proper position relative to the weld head.
Some prior art pipe collets, designed to accommodate a significant amount of size variation, use a series of slots that are cut into the base collet material to form resilient fingers. The slots can be cut either radially outward from the theoretical center of the object to be held, or they can be cut tangentially with respect to that theoretical center. The geometry of these slots enable the collet fingers to flex, or bend, in response to the geometry of the pipe that is being clamped within the collet. Where a large amount of compliance is not required, the collets can be left solid, that is, without machined slots. Some manufacturers cut the inside diameters of these collets on the true center of the collet, and some manufacturers offset the ID cut.